Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


Hidden fault may contribute to Bay Area earthquake risk


Figure 1. Tectonic setting of region of proposed study. Generalized location of major fault strands through the San Francisco Bay region are shown by dashed lines. Primary study area is outlined by bos. Hypothetical blind thrusts are schematically indicated by ’dashed’ thrust fault symbols - are purely schematic at present. Mt. Tamalpais and Bolinas Ridge are identified
Credit: Penn State, Kevin Furlong

Earthquakes are not unusual in the San Francisco Bay Area, but a team of Penn State geoscientists believes that the hazard may be greater than previously thought because of a hidden fault under Marin County.

"We think we have evidence that there is an additional earthquake hazard in the San Francisco area due to a blind thrust fault," says Dr. Kevin P. Furlong, professor of geosciences. "Blind thrust faults are notorious because they are hard to find until an earthquake occurs on them. A blind thrust fault caused the 1994 Northridge earthquake."

The San Francisco Bay Area has a variety of known faults running through it. The San Andreas fault runs on the west, while the Hayward fault is on the east and shifts into the Rodgers Creek fault northeast of the city. The San Gregorio fault, west of the San Andreas, meets that fault near the Golden Gate Bridge.

The slip rate on the Hayward fault is about 9 millimeters a year and the slip on the Rodgers Creek fault is probably 6 to 9 millimeters a year, according to Furlong. These two rates are consistent. However, the slip on the San Andreas fault south of the Golden Gate Bridge is 17 millimeters per year and the slip rate north of the bridge is 24 millimeters per year. Adding in the San Gregorio fault slip of about 3 millimeters a year, the slip rate on the northern San Andreas is still inconsistent with the southern portion of the fault by as much as 4 millimeters per year.

Compounding the geological confusion is the existence of Mt. Tamalpais at 2,640 feet sitting northeast of San Francisco in Marin County. Geologists are unsure why Mt. Tamalpais is there and what formed it.

Furlong and Dr. Eric Kirby, assistant professor of geosciences, looked for an explanation for the different slip rates on the San Andreas fault north and south of the Golden Gate that would also explain how Mt Tamalpais came to rise above the bay.

"In the past, the thought was that there must be something wrong with the calculations on the San Gregorio fault," says Furlong. "That is, the slip rate must be closer to 7 millimeters per year because we do not see any other faults."

The researchers hypothesized that a fault running diagonally from the northern Hayward fault to the San Andreas in Marin County could transfer the necessary motion to the San Andreas and might explain high topography around Mt Tamalpais. However, proving the fault’s existence turns out to be a difficult task.

Thrust faults occur when one piece of terrain rides up over another forming a characteristic uplift pattern with one side gradually sloping up and the other more precipitous. Blind thrust faults terminate below the Earth’s surface and are therefore blind.

"We asked, can we demonstrate that Mt Tamalpais is currently going up, thrusting?" said Furlong. "Uplift rates are just at the limit of what can be measured with geographic positioning systems and there are not sufficient prior measurements on the mountain to compare with anyway," he told attendees at the fall conference of the American Geophysical Union in San Francisco.

Estimating that the uplift is about 1 millimeter per year – less than normally observable with GPS, the researchers looked for another way to measure uplift. They considered the topography of Marin County. Bolinas Ridge – just east of Point Reyes – runs up the west side of the area just to the west of Mt. Tamalpais and has a gradually rising slope, similar to the uplift predicted by a blind fault. It also has numerous streams running off the ridge peak into the San Andreas fault.

"If an area is uplifting, then we typically see steep rivers, but if the uplift is slow or nonexistent, then we see gentle rivers," says Furlong. "What we find is that the rivers become substantially steeper in the southern portion of Bolinas Ridge, implying that there may be active uplift in the area."

The rock making up Bolinas Ridge is of uniform composition, so differences in river slopes probably reflect differences in erosion rate. Assuming there is a blind thrust fault, the slip rate on the fault would need to be 3 to 4 millimeters to even out the discrepancies north and south of the Golden Gate Bridge. If the slip is 3 to 4 millimeters, then the researchers calculate that there would be potential for an earthquake of magnitude 6 to 6.5 on the fault occurring on a time scale of several hundred years.

Courtney B. Johnson, graduate student in Penn State’s department of geosciences, is presenting a poster that further explores the size and earthquake potential of the blind thrust fault.

"No large earthquakes have occurred in that area for at least the 150 to 200 years we have records," says Furlong. "In fact, there are very few earthquakes of any size in this general area."

The researchers are planning research to prove that the blind thrust fault exists. Unfortunately, standard seismic imaging will not work on the type of rock in the area, so alternative methods are necessary.

"The Loma Prieta earthquake did a lot of damage to the East Bay and Marina areas of San Francisco," says Furlong. "An earthquake in Marin County, while smaller, is much closer and is of concern not only to those in the county, but also to that part of San Francisco."

A’ndrea Elyse Messer | EurekAlert!
Further information:

More articles from Earth Sciences:

nachricht Receding glaciers in Bolivia leave communities at risk
20.10.2016 | European Geosciences Union

nachricht UM researchers study vast carbon residue of ocean life
19.10.2016 | University of Miami Rosenstiel School of Marine & Atmospheric Science

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: New 3-D wiring technique brings scalable quantum computers closer to reality

Researchers from the Institute for Quantum Computing (IQC) at the University of Waterloo led the development of a new extensible wiring technique capable of controlling superconducting quantum bits, representing a significant step towards to the realization of a scalable quantum computer.

"The quantum socket is a wiring method that uses three-dimensional wires based on spring-loaded pins to address individual qubits," said Jeremy Béjanin, a PhD...

Im Focus: Scientists develop a semiconductor nanocomposite material that moves in response to light

In a paper in Scientific Reports, a research team at Worcester Polytechnic Institute describes a novel light-activated phenomenon that could become the basis for applications as diverse as microscopic robotic grippers and more efficient solar cells.

A research team at Worcester Polytechnic Institute (WPI) has developed a revolutionary, light-activated semiconductor nanocomposite material that can be used...

Im Focus: Diamonds aren't forever: Sandia, Harvard team create first quantum computer bridge

By forcefully embedding two silicon atoms in a diamond matrix, Sandia researchers have demonstrated for the first time on a single chip all the components needed to create a quantum bridge to link quantum computers together.

"People have already built small quantum computers," says Sandia researcher Ryan Camacho. "Maybe the first useful one won't be a single giant quantum computer...

Im Focus: New Products - Highlights of COMPAMED 2016

COMPAMED has become the leading international marketplace for suppliers of medical manufacturing. The trade fair, which takes place every November and is co-located to MEDICA in Dusseldorf, has been steadily growing over the past years and shows that medical technology remains a rapidly growing market.

In 2016, the joint pavilion by the IVAM Microtechnology Network, the Product Market “High-tech for Medical Devices”, will be located in Hall 8a again and will...

Im Focus: Ultra-thin ferroelectric material for next-generation electronics

'Ferroelectric' materials can switch between different states of electrical polarization in response to an external electric field. This flexibility means they show promise for many applications, for example in electronic devices and computer memory. Current ferroelectric materials are highly valued for their thermal and chemical stability and rapid electro-mechanical responses, but creating a material that is scalable down to the tiny sizes needed for technologies like silicon-based semiconductors (Si-based CMOS) has proven challenging.

Now, Hiroshi Funakubo and co-workers at the Tokyo Institute of Technology, in collaboration with researchers across Japan, have conducted experiments to...

All Focus news of the innovation-report >>>



Event News

#IC2S2: When Social Science meets Computer Science - GESIS will host the IC2S2 conference 2017

14.10.2016 | Event News

Agricultural Trade Developments and Potentials in Central Asia and the South Caucasus

14.10.2016 | Event News

World Health Summit – Day Three: A Call to Action

12.10.2016 | Event News

Latest News

Resolving the mystery of preeclampsia

21.10.2016 | Health and Medicine

Stanford researchers create new special-purpose computer that may someday save us billions

21.10.2016 | Information Technology

From ancient fossils to future cars

21.10.2016 | Materials Sciences

More VideoLinks >>>